Studies are continuing on characterization of the lectin-carbohydrate interaction that mediates lactose sensitive coaggregation beween oral actinomyces and streptococci. The lectin activity is a property of the type 2 fimbriae on strains of Actinomyces viscosus and A. naeslundii with lectin receptors on specific strains of Streptococcus sanguis and S. mitis. The introduction of A. viscosus chromosomal DNA into Escherichia coli using recombinant DNA technology and the isolation of clones expressing specific actinomyces antigens has provided an approach for the isolation of unassembled fibrial subunits or precursor molecules. One clone has been found to produce a protein of approximately 60,000 daltons that reacts with monospecific and several monoclonal antibodies against type 2 fimbriae. A protein of similar size and antigenicity is present in A. viscosus cells and also has been detected as a subunit of partially dissociated type 2 fimbriae. An additional protein of approximately 35,000 daltons may also be involved in formation of the type 2 structure. In addition to studies of the type 2 fimbriae, immunochemical studies of S. sanguis 34 have associated a specific carbohydrate antigen with the receptor structure for lactose sensitive coaggregation. This antigen was detected in a mutanolysin digest of S. sanguis 34 cell walls but was not present on a spontaneous coaggregation defective mutant selected by its failure to react with certain D-galactose and N-acetyl-D-galactosamine binding plant lectins. The in vivo interaction of this receptor associated antigen with the type 2 fimbriae of actinomyces has been inferred from fluorescent double labeling of plaque samples using specific antibodies. Close and specific associations have been observed between actinomyces with type 2 fimbriae and streptococci labeled by monospecific antibody against the lectin receptor of S. sanguis 34. Such observations strongly support the concept that interbacterial adherence mediated by lectin-carbohydrate interactions is a central mechanism in the microbial colonization of teeth.